This invention is directed to a novel battery charger utilizing a constant current source and speed up circuit and which includes automatic charging current reduction as the battery becomes fully charged.
Battery chargers using high speed pulse width modulation (PWM) switching of constant current sources are well-known in the art for supplying current for charging batteries. High speed transistor switching circuits, particularly those used with single-ended power supplies, are limited by the time required to remove base current from the switch transistor. In double-ended power supplies, i.e., those providing positive and negative operating potentials, the problem of rapidly removing base current is simplified as compared with single-ended supplies where one side of the supply is held at ground potential.
A speed up circuit, claimed in application Ser. No. 272,194 above, is used in the present invention battery charger to rapidly remove the base drive to the switch transistor and thereby result in a dramatic enhancement of its switching characteristics. The decrease in switching time enables a significant reduction in heat dissipation in the transistor and permits utilization of a much smaller heat sink.
In many applications, such as in portable television receivers and computers where the batteries comprise a very large part of the overall device cost, it is desirable to use rechargeable battery packs. Rechargeable battery packs generally have nickel cadmium batteries, the useful lives of which are highly dependent upon the manner in which they are recharged. It is common for a manufacturer to guarantee a battery pack for a minimum number of discharge and charge cycles, provided that the batteries are recharged under controlled conditions. Nickel cadmium batteries are best charged either with a low continuous current or with a larger constant current until a certain battery temperature is attained. To do otherwise can have an adverse effect on their useful lives. Consequently, the equipment used to recharge the battery pack must be carefully designed. As a battery recharging circuit is often built into the powered device, space and weight are at a premium and a high efficiency recharging circuit is desirable.
The present invention is directed to a switching type recharging circuit for supplying a constant RMS current to a battery pack and includes an automatic current reduction or "foldback" circuit for automatically reducing the charging current as the battery reaches a fully charged state. For the battery pack used, the manufacturer specifies either a charging rate of 1.2 amps until a case temperature of 55.degree. C. is attained, or a continuous charge of 200 milliamps. The circuit constructed in accordance with the invention accomplishes this with very high efficiency, on the order of 84%, without a speed up circuit. With the speed up circuit of the copending application, an efficiency in excess of 90% is attained. Further, the current supplied to the battery changes less than 1% from discharge to full charge and thereafter idles at the recommended minimum level.